You must do something. You cannot avoid doing it. If you do not do it, there will be problems.
Verb [ない-form] + ではすまないPrimary
You must do something. You cannot avoid doing it. If you do not do it, there will be problems.
部下の大きなミスに対して、上司である私が謝らないではすまないだろう。
Regarding my subordinate's major mistake, as their superior, I will have no choice but to apologize.
店の窓ガラスを割ってしまったので、弁償しないではすまなかった。
Since I broke the store's windowpane, I couldn't get away with not compensating for it.
国民の信頼を裏切った以上、責任を取って辞職しないではすまない、と多くの人が考えている。
Many people believe that having betrayed the trust of the nation, he has no choice but to take responsibility and resign.
親友が道を踏み外しそうになっている。このままでは、厳しい忠告をしないではすまない。
My best friend is about to go astray. If things continue like this, I'll have no choice but to give him some harsh advice.
これだけお世話になったのだから、ちゃんとお礼をしないではすまない。
Having received this much help, I can't possibly get away with not expressing my gratitude properly.
This grammar shows you must do something. If you don't, things will not end well. People might judge you. Or the problem will not go away.
This is different from "〜ざるを得ない" (zaru-o-enai). "〜ざるを得ない" means you have no other choice. You must do something, even if you do not want to. "〜ないではすまない" means you must do something because it is the right thing to do. If you do not do it, it will be bad. It is about what society expects, not just having no other option.
Don't use this when you talk about feelings or states, like "I must be happy." You use it for things you have to do.
Less formal alternatives(1)
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